1. Field of the Invention
The present invention relates to an apparatus for guiding the air current in a wafer loading chamber for chemical vapor deposition equipment, and in particular to an apparatus having an improved air intake unit to supply ultra-clean air to a boat containing wafers before or after a low pressure chemical vapor deposition process.
2. Background of the Related Art
Generally, semiconductor wafers are used, for among others, Read-Only Memory (ROM), Random-Access Memory (RAM), and Integrated Circuits (IC). The wafer becomes ROM or RAM of a certain memory capacity, or an IC containing particular circuits, after conventional oxidation, diffusion, deposition, etching, and packaging processes.
Chemical Vapor Deposition (hereinafter, referred to as CVD) is the most widely adopted technology for the deposition process on the wafer. CVD may be classified into Atmospheric Pressure CVD (hereinafter, referred to as APCVD), Low Pressure CVD (hereinafter, referred to as LPCVD), and Plasma Enhanced CVD (hereinafter, referred to as PECVD). LPCVD is generally used to produce MOS integrated circuits, charge coupled devices (hereinafter, referred to as CCDS), and various transistors. LPCVD has the following characteristics: outstanding uniformity in thickness and electric resistance between wafers and in the wafer; reduced natural doping from the reaction chamber to a wafer because of the low temperature and pressure; and economical operation because many wafers can be processed simultaneously. LPCVD also maintains the distribution of impurities formed in the wafer before the deposition process because it utilizes a low temperature process. In view of these above listed characteristics, LPCVD is used to form a nitride or an oxidized film on a wafer.
Generally, an LPCVD apparatus is divided into a reaction chamber and a wafer loading chamber. The reaction chamber, having a certain inside temperature and ambience, is an apparatus that forms a thin film on wafers loaded on a boat. The wafer loading chamber is an apparatus that loads wafers for deposition of a film on the boat and then transports them to the reaction chamber, or unloads wafers after the deposition process on the boat. There is also an apparatus for supplying gases such as SiH.sub.4, N.sub.2 O, PH.sub.3, and N.sub.2, and a ventilation system to maintain clean internal air and to prevent it from contamination due to impure particles.
An upright LPCVD apparatus generally has a loading chamber installed under a reaction chamber. The reaction chamber is equipped with a gas intake device. The wafer loading chamber has a device for supplying and discharging clean air to shower the wafers with air to prevent their contamination by impure particles.
FIG. 1A is a top view of an upright LPCVD apparatus and FIG. 1B is a side view of the same. A robot (not shown) placed in the wafer loading chamber loads wafers onto a boat 12 or moves wafers on a boat 12 to a carrier in order to transport wafers in lot units for subsequent processes. The boat 12 is installed in the wafer loading chamber and moves up and down in order to load many wafers into the reaction chamber for deposition. A filter 14 for the filtration of incoming air is on a wall and a plurality of outlets 16 are on the opposing wall to discharge inside air to an outside exhaust device (not shown).
In the LPCVD apparatus constructed as in FIG. 1A and FIG. 1B, clean air should be supplied into the wafer loading chamber 10 at a velocity of 0.2-0.4 m/s for effective removal of impurities on a wafer. However, the surface area of the filter 14 is not large enough for the inside capacity of the wafer loading chamber 10 in the conventional upright LPCVD apparatus shown in FIG. 1A and FIG. 1B. Also, the wafers on the boat 12 are not placed directly in front of the air inlet and, in consequence, the air flowing in from the filter 14 does not proceed directly to the wafers on the boat 12, but rather forms an air current as marked by the arrows in FIG. 1A. Therefore, small particles from other areas such as the robot may contaminate the wafers through the air current.
The air is especially turbulent near the boat 12 because of the improper placement of outlets 16, and the air turbulence interferes with the removal of impurities. The number of particles per unit area is about 100-300 according to measurements, and thus these particles may contaminate the wafers.
The air path after the filter 14 is also affected by the turbulence. That is, near the boat 12, which is a sensitive area for the impurity removal from wafers, a dead point 18, namely, the point where the air current speed is abruptly reduced down to 0-0.2 m/s, is formed and, in consequence, the air current after the dead point does not contribute to the cleanliness of the wafers and the boat.
To solve the above problems in the prior art, the size and position of the filter 14 must be changed accordingly. However, it is not easy to modify or replace the filter installation because of the substantial cost involved with structural modifications of expensive semiconductor manufacturing instruments that are already set up. Also, modifications to the lay-out of the installation is not desirable due to structural correlations between instruments.